don't report type / term aliases as ambiguous

src/compiler/scala/tools/nsc/typechecker/Contexts.scala

@@ -1455,7 +1455,7 @@ trait Contexts { self: Analyzer =>
       }
 
       // cx.scope eq null arises during FixInvalidSyms in Duplicators
-      def nextDefinition(): Unit = {
+      def nextDefinition(lastDef: Symbol, lastPre: Type): Unit = {
         var inPrefix = false
         defSym = NoSymbol
         while (defSym == NoSymbol && (cx ne NoContext) && (cx.scope ne null)) {
@@ -1466,10 +1466,14 @@ trait Contexts { self: Analyzer =>
           }
           if (!defSym.exists) cx = cx.outer // push further outward
         }
+        if ((defSym.isAliasType || lastDef.isAliasType) && pre.memberType(defSym) =:= lastPre.memberType(lastDef))
+          defSym = NoSymbol
+        if (defSym.isStable && lastDef.isStable && singleType(pre, defSym) =:= singleType(lastPre, lastDef))
+          defSym = NoSymbol
         foundInPrefix = inPrefix && defSym.exists
         foundInSuper  = foundInPrefix && defSym.owner != cx.owner
       }
-      nextDefinition()
+      nextDefinition(NoSymbol, NoPrefix)
 
       if (symbolDepth < 0)
         symbolDepth = cx.depth
@@ -1581,7 +1585,7 @@ trait Contexts { self: Analyzer =>
           while ((cx ne NoContext) && (cx.owner eq cx0.owner)) cx = cx.outer
         var done = false
         while (!done) {
-          nextDefinition()
+          nextDefinition(defSym0, pre0)
           done = (cx eq NoContext) || defSym.exists && foundCompetingSymbol()
           if (!done && (cx ne NoContext)) cx = cx.outer
         }

test/files/neg/t11921-alias.check

@@ -0,0 +1,15 @@
+t11921-alias.scala:16: error: reference to TT is ambiguous;
+it is both defined in object O and available as type TT in class C
+Since 2.13.11, symbols inherited from a superclass no longer shadow symbols defined in an outer scope.
+If shadowing was intended, write `this.TT`.
+Or use `-Ylegacy-binding` to enable the previous behavior everywhere.
+      def n(x: TT) = x // ambiguous
+               ^
+t11921-alias.scala:36: error: reference to c is ambiguous;
+it is both defined in class B and available as value c in class A
+Since 2.13.11, symbols inherited from a superclass no longer shadow symbols defined in an outer scope.
+If shadowing was intended, write `this.c`.
+Or use `-Ylegacy-binding` to enable the previous behavior everywhere.
+      def n = c // ambiguous
+              ^
+2 errors

test/files/neg/t11921-alias.scala

@@ -0,0 +1,39 @@
+object t1 {
+  class C[T] { type TT = T }
+  object O {
+    type TT = String
+    class D extends C[TT] {
+      def n(x: TT) = x // OK
+    }
+  }
+}
+
+object t2 {
+  class C[T] { type TT <: T }
+  object O {
+    type TT = String
+    class D extends C[TT] {
+      def n(x: TT) = x // ambiguous
+    }
+  }
+}
+
+object t3 {
+  trait Context
+  class A[C <: Context](val c: C)
+  class B(val c: Context) { b =>
+    val a = new A[c.type](c) {
+      def n = c // OK
+    }
+  }
+}
+
+object t4 {
+  trait Context
+  class A[C <: Context](val c: C)
+  class B(val c: Context) { b =>
+    val a = new A(c) {
+      def n = c // ambiguous
+    }
+  }
+}